The present tendency of development as regards pistons for internal combustion engines, in particular those such as diesel engines which are subjected to an elevated thermal loading, is directed mainly to two basic criteria, namely the pursuit of increased levels of engine performance, and a reduction in weight. A major concern of manufacturers is thus to resolve the problems raised by increasing compression ratios and increasing engine outputs, as well as the possible addition of supercharging systems, but without increasing the weight of the engine.
The piston remains one of the components of the engine which is the most exposed to very severe heat stresses, combined with mechanical forces, which are a necessary result from such developments. This therefore raises difficult problems as regards the service life of the pistons, particularly in dealing with the high temperatures generated at the pistons, cooling the piston ring areas of the pistons, and achieving the optimum coefficient of strength at the most highly critical places of the piston.
The above problems tend to affect the performance of the piston. However the rising increases in engine output gives rise to another danger, in so far as the engine manufacture who wishes to increase the output of the engine is naturally led to reduce the stroke of the piston, which also generally has the advantage of reducing the space occupied by the engine, and also its weight. Using a conventional expression in the art, engines are tending to become "square" or even "over-square," that is to say, the bore-stroke ratios adopted are coming closer and closer to and even exceeding unity. The piston inevitably suffers from the repercussions of such changes, in that the height of the piston above the small end connection and also the total height of the piston decrease. Under these conditions, the small end connection between the connecting rod and the piston is brought very close to the head or top face of the piston, where the temperatures are at their highest. The oscillating movement of the small end shaft therefore occurs in a very testing medium, which can involve serious dangers of phenomena such as rupture or seizing.